Are you getting any error/test send failed reports to your inbox? Are you able to replicate the issue if you try to send a different email? Sometimes sends will fail if the email is broken in some way but this usually triggers an error notification to the target inbox.
Personally, I would contact Marketo support about this; it sounds like a bug - especially if you're not getting error reports.
We aren't receiving any errors/test send failed reports to our inbox. If we try to send from a different email we are getting the same result. We will contact Marketo Support. Thanks!
Does your email live in Design Studio? Does it use My Tokens (which live inside of programs/folders)?